To enable a long service life, it is important that a bearing is properly lubricated. Most bearings are grease lubricated, whereby oil from the grease generates an oil film during bearing operation, which separates the rolling contact surfaces. If the oil film breaks down, metal-to-metal contact occurs that can quickly lead to bearing failure if the oil film is not replenished. Monitoring methods are therefore applied in some applications to monitor the lubrication condition in a bearing, so that re-lubrication can be carried out before failure occurs. In EP1676041, for example, capacitance measurements are used to determine the film thickness of the oil film.
Lubricant contamination can also lead to early bearing failure. If an oil or grease lubricant contains abrasive particles, the rolling contact surfaces are likely to get damaged. The presence of water is also undesirable, as this can lead to surface corrosion. Water can also accelerate chemical degradation of a lubricating oil or of the thickener structure within a grease.
Therefore, in applications where a bearing might be exposed to the ingress of contamination, condition monitoring of the lubricant is sometimes applied. Capacitance measurement can be used to detect contamination. JP 2007198576, for example, discloses a sealed bearing, whereby a pair of electrodes is provided on an inner, axially oriented surface of a seal, for detecting the ingress of water.